1. Field of the Invention
This invention relates to a portable power unit which generates a single-phase AC power having a commercial frequency or a like frequency, and more particularly to a portable power unit of this kind which uses a cycloconverter.
2. Prior Art
Conventionally, a portable power unit which is a combination of a small-sized engine and a synchronous generator, for instance, is widely used for emergency purposes, outdoor works, leisure time amusement, etc.
In this type of conventional portable power unit, however, the output frequency depends on the rotational speed of the engine. Therefore, in the case of a bipolar generator, to obtain an AC output of 50 Hz (or 60 Hz), the rotational speed of the engine is required to be held at 3000 rpm (or 3600 rpm), i.e. a relatively low rotational speed, which degrades the operating efficiency of the power unit, and further, necessitates designing the generator to be large in size, resulting in an increased total weight of the power unit.
To overcome this inconvenience, a so-called inverter generator has been proposed by the present assignee, e.g. in Japanese Patent Publication (Kokoku) No. 7-67229 and Japanese Laid-Open Patent Publication (Kokoku) No. 4-355672, in which an engine is operated at a relatively high rotational speed, to obtain a high AC power from a generator, and the AC power is once converted to direct current, and then to alternating current having a commercial frequency by an inverter.
The conventional inverter generator, however, requires provision of two power conversion blocks, i.e. an AC-to-DC conversion block for once converting the AC power to DC power, and a DC-to-AC conversion block for converting the DC power to AC power having a predetermined frequency, as well as a circuit for temporarily storing the DC power. Thus, the use of a lot of expensive power circuit components is necessitated. This makes it difficult to reduce the size of the generator and leads to an increased manufacturing cost.
On the other hand, a so-called cycloconverter is conventionally known, which directly converts AC power with a fixed frequency to AC power with another frequency. The cycloconverter is employed in power plants.
The conventional cycloconverter is normally used for converting power supplied from a commercial frequency power line or power generated by a high power generator (see Japanese Patent Publication (Kokoku) No. 60-9429, for instance), and it is generally used for driving an AC electric motor.
The operating principle of the cycloconverter will be described with reference to FIGS. 1 to 6.
FIG. 1 is a circuit diagram showing an example of the construction of the conventional cycloconverter.
As shown in the figure, this cycloconverter CC is comprised of twelve thyristors SCRk.+-. (k=1, 2, . . . , 6), with six thyristors SCRk+ thereof forming a bridge circuit (hereinafter referred to as "the positive converter") BC1 for delivering positive electric current, and the remaining six thyristors SCRk- thereof forming another bridge circuit (hereinafter referred to as "the negative converter") BC2 for delivering negative electric current. In other words, the two bridge circuits are connected in antiparallel connection to each other to form the cycloconverter.
When a three-phase generator with 27 poles (three of them are used to generate synchronizing signals for control of respective gates of the thyristors SCRK.+-.), for instance, is connected to the cycloconverter CC, and driven by an internal combustion engine, nine cycles of three-phase alternating current are supplied to the cycloconverter per one revolution of the crankshaft of the engine. If the rotational speed of the engine is set to a range of 1200 rpm to 4500 rpm (equivalent to a frequency range of 20 Hz to 75 Hz), the frequency of the three-phase AC output from the generator is 180 Hz to 675 Hz, nine times as high as the rotational speed of the engine.
Components of the three-phase alternating current (i.e. U-phase current, V-phase current, and W-phase current) obtained from coils of the above-mentioned three poles (these coils will be hereinafter referred to as "the sub coils" and the coils of the remaining poles as "the main coils") are supplied to a three-phase full-wave bridge rectifier FR formed by primary light-emitting diodes (LED's) of respective six photocouplers PCk (k=1, 2, . . . , 6) and six diodes Dk (k=1, 2, . . . , 6), as shown in FIG. 2. Direct current components of the three-phase alternating current full-wave rectified by the three-phase full-wave rectifier FR are each transformed into light by a corresponding one of the primary light-emitting diodes, and then the light is converted into electric current by a corresponding one of secondary photosensors, not shown, associated with the primary light-emitting diodes of the photocouplers PCk. In short, electric currents corresponding to the three-phase alternating current components full-wave rectified by the three-phase full-wave rectifier FR are delivered from the secondary photosensors of the photocouplers. These electric currents are each used to form a synchronizing signal having e.g. a sawtooth waveform for controlling a phase control angle (firing angle) a of a gate of each of the thyristors SCRk.+-., as described in detail hereinafter.
FIG. 3 shows changes in line-to-line voltages appearing between respective pairs of the U, V, and W phases of the three-phase AC power and timing of "turn-on" of each photocoupler PCk.
Assuming that the line-to-line voltages (U-V, U-W, V-W, V-U, W-U, and W-V) change as shown in FIG. 3, the waveform of a full-wave rectified output from the three-phase full-wave rectifier FR has a repetition period of one sixth of that of the waveform of each line-to-line voltage obtained from the main coils. For example, when the U-V voltage is in a phase angle range of 60.degree. to 120.degree. where the U-V voltage is the highest of all the line-to-line voltages, the photocouplers PC1 and PC5 are turned on in pair (the other photocouplers are held off), whereby the three-phase full-wave rectifier circuit FR delivers electric current at a voltage corresponding to the U-V voltage. That is, the three-phase full-wave rectifier FR delivers electric current at a voltage corresponding to the maximum value of all the line-to-line voltages, so that the repetition period of the output voltage corresponds to a phase angle of 60.degree., and hence is equal to one sixth of the repetition period of the three-phase output voltage of the main coils, which corresponds to a phase angle of 360.degree..
FIG. 3 also shows a controllable range of timing of firing (turn-on) of the gate of each of the thyristors SCRk.+-., which is set to a phase angle range of 120.degree. to 0.degree. of a corresponding line-to-line voltage with two examples of timing of firing of each gate which are indicated by hatched portions (i.e. firing angles of 120.degree. and 60.degree.) described hereinafter.
According to this timing, each gate of the positive converter BC1 is fired (turned on) to deliver electric current therefrom, and each gate of the negative converter BC2 is turned on to absorb electric current thereto.
Needless to say, the gates are not required to be continuously held on over a selected portion of the controllable range, but the application of a predetermined pulse at timing indicated by the hatched portion (e.g. corresponding to one of the firing angles of 120.degree. and 60.degree.) enables the same operation as above to be performed.
FIGS. 4A to 4D shows examples of waveforms of the output of the cycloconverter obtained when the thyristors SCRk.+-. of the positive and negative converters BC1 and BC2 are fired at respective firing angles of 120.degree. and 60.degree..
FIG. 4A shows an output waveform of the cycloconverter CC obtained when each thyristor SCRk+ of the positive converter BC1 is turned on at a firing angle .alpha. of 120.degree., and FIG. 4B an output waveform of the same obtained when each thyristor SCRk- of the negative converter BC2 is turned on at a firing angle .alpha. of 120.degree.. On the other hand, FIG. 4C shows an output waveform of the same obtained when each thyristor SCRk+ of the positive converter BC1 is turned on at a firing angle .alpha. of 60.degree., and FIG. 4D an output waveform of the cycloconverter CC obtained when each thyristor SCRk- of the negative converter BC2 is turned on at a firing angle .alpha. of 60.degree..
When each thyristor SCRk+of the positive converter BC1 is turned on at the firing angle .alpha. of 120.degree., the output waveform of the cycloconverter CC presents a full-wave rectified current waveform as shown in FIG. 4A. When each thyristor SCRk+ of the positive converter BC1 is turned on at a firing angle .alpha. of 60.degree., the output waveform contains a lot of harmonic components as shown in FIG. 4C. These harmonic components, however, can be removed by a low-pass filter connected to the output side of the cycloconverter CC, so that electric current is output at an averaged voltage. As described hereinabove, assuming that the power supply to the cycloconverter is a three-phase generator having 27 poles, and the rotational speed of the engine is set to 3600 rpm, the frequency of a basic wave of the harmonic components is given by the following equation: EQU 60 Hz (=3600 rpm).times.9(-th harmonic).times.3(phases).times.2(half waves (=1 full wave))=3.24 kHz
Further, by varying the firing angle .alpha. of each thyristor of the positive converter BC1 within a range of 0.degree. to 120.degree., the cycloconverter CC is capable of generating a positive voltage as desired which has an average voltage within a range of 0 V to a positive full-wave rectified voltage. By varying the firing angle .alpha. of each thyristor of the negative converter BC2 in the same manner, the cycloconverter CC is capable of generating a negative voltage as desired which has an average voltage within a range of 0 V to a negative full-wave rectified voltage.
Next, the manner of varying the firing angle a within the range of 0.degree. to 120.degree. will be described.
FIG. 5 shows reference sawtooth waves generated for controlling the firing angle .alpha. of each thyristor of the cycloconverter. The reference sawtooth waves shown in the figure are generated based on respective electric currents detected by i.e. taken out from the secondary photosensors of the photocouplers.
A reference sawtooth wave for control of the thyristor SCR1+ of the positive converter BC1, for instance, is one which changes in voltage within a phase angle range of 120.degree. to 0.degree. and assumes 0 V at a phase angle of 0.degree.. Reference sawtooth waves each having a phase difference of 60.degree. from adjacent ones sequentially correspond to the thyristors SCRk+ , i.e. SCR1+, SCR6+, SCR2+, SCR4+, SCR3+, and SCR5+, respectively.
On the other hand, a reference sawtooth wave for control of the thyristor SCR1- of the negative converter BC2, for instance, is one which is symmetrical with the sawtooth wave for the thyristor SCR1+ with respect to a horizontal zero voltage line, i.e. which has a phase difference of 180.degree. from the sawtooth wave for the thyristor SCR1+. Similarly to the positive converter BC1, reference sawtooth waves each having a phase difference of 60.degree. from adjacent ones sequentially correspond to the thyristors SCRk-, i.e. SCR1-, SCR6-, SCR2-, SCR4-, SCR3-, and SCR5-, respectively.
Thus, the twelve sawtooth waves provide respective reference waveforms for control of the thyristors SCRk.+-. of the positive and negative converters BC1, BC2. These sawtooth waves are compared with a desired waveform r by the use of comparators, not shown, provided in twelve channels, and a point of intersection of each sawtooth wave with the desired waveform (e.g. a point T0 in the case of the thyristor SCR1+) determines a firing angle of each corresponding thyristor SCRk.+-..
By employing a sinusoidal wave as the desired wave to thereby sinusoidally varying the firing angle .alpha., it is possible to obtain a sinusoidal output wave from the cycloconverter CC, as shown in FIG. 6. When a sinusoidal output wave of 50 Hz is obtained from the input waves each having a frequency of 540 Hz, for example, the output wave is fabricated from approximately 65 portions of the input sinusoidal waves connected one after another.
In the conventional power unit using the cycloconverter described above, the cycloconverter is not provided with means for storing energy. Therefore, when the cycloconverter is used to obtain single-phase sinusoidal alternating current, the energy input to the cycloconverter also sinusoidally changes.
Therefore, when a small-sized generator which generates small power e.g. of several hundreds to several thousands kW is connected to the input side of the cycloconverter to generate a single-phase sinusoidal wave, only portions of the input sinusoidal waves can be utilized as input energy, so that the utilization efficiency is very low, resulting in only a very small output power being taken out as single-phase alternating current.
Particularly, when a magneto generator is employed as the small-sized generator, if an electric motor as a load is connected to the power unit, the limited output power of the magneto generator can cause the power unit to become excessively loaded and inoperative due to even temporary large electric current which flows when the electric motor is started.
Further, in the conventional cycloconverter CC, when a small-sized generator which generates several hundreds to several thousands kW is connected to the output side of the cycloconverter, a large output voltage drop occurs when a heavy load is connected to the power unit, due to the limited power-generating capacity of the generator. The output voltage drop is particularly large when a magneto generator is employed as the generator, raising the following problem:
FIG. 7 shows voltages applied to the thyristors SCRk.+-., when a 230-volt alternating current output is obtained from the cycloconverter CC. In the figure, G designates a magneto generator used as the generator.
As mentioned above, the magneto generator has a load characteristic that the output voltage has a large drop relative to a load applied to the generator. Therefore, to obtain an AC 230 V output from the cycloconverter CC, the line-to-line voltage should exhibit a peak value as high as 600 Vp when the power unit is in a no-load condition. Assuming that the thyristors SCR1+ and SCR6+ are turned on in pair and the output voltage assumes a peak value of AC 230 V, as shown in the figure, the voltage Vscr applied to the thyristor SCR5+ can be calculated by the following equation: EQU Vscr=230.times..sqroot.2 Vp+600 Vp=920 V
In general, small-sized thyristors available have a withstand voltage of approximately 600 V at the maximum. Therefore, in conventional portable power units, so long as such small-sized thyristors are used, it is impossible to obtain AC 230 V power from the cycloconverter.